Ceramic paint compositions are generally well known to the skilled artisan. These ceramic paint compositions can be used, for example, to form borders around the edges of glass sheets which are used as windshields, side lites, and back lites in motor vehicles.
Such ceramic paints usually include a mixture of metal oxides which together act as a coloring agent for the ceramic paint. The metal oxides are nonreactive with one another and nonreactive with any elements or compounds they come in contact with while being heated to temperatures up to about 1300.degree. F. The mixture of metal oxides can be controlled so as to get a selected color from the ceramic paint composition. Normally, in automotive applications, the selected color is black, although shades of grey are now becoming popular.
Such ceramic paint compositions also include a low melting point glass frit which generally melts at a temperature below 1300.degree. F. The low melting point glass frit is the material which bonds the mixture of metal oxides to the glass sheet and ensures that it remains after the glass sheet has been cooled back to room temperature. A vehicle is normally mixed with the metal oxides and low melting point glass frit to allow the ceramic paint composition to be applied in a paint application process. For example, if the paint application process is a silk screen printing operation, the vehicle is an oil-base organic medium for carrying the other materials during the silk screen printing operation.
Many different types of ceramic paint compositions of the above general type are well known to skilled artisans. The exact metal oxides, low melting point glass frit, and oil-base vehicle which are selected for such a paint, are well within the skill of artisans in this area. The manner in which the different materials may be varied in order to achieve the results desired in a particular application is also well within the skill of an artisan.
As was discussed in my recently issued patent, U.S. Pat. No. 4,596,590, I found that, in the situation where a sheet having a ceramic paint thereon was contacted by a fiberglass forming die, certain paints of the above-described general composition had a great tendency to stick on the fiberglass covered die. The sheet of glass, of course, was being heated to an elevated temperature, generally less than 1300.degree. F., so that it may be engaged by a forming die and formed into a configuration other than flat.
As further discussed in my patent '590, the difficulty arose that the ceramic paint compositions generally known to skilled artisans would not operate in a glass forming process in which a heated glass sheet containing the ceramic paint composition was engaged by a fiberglass covered forming die. As I disclosed in my patent, no other materials are known for covering the forming die because the fiberglass is the only material which will withstand the heat and rigorous requirements of the environment to which it is subjected. Other materials that have been used to cover forming dies quickly break down and wear out when repeatedly brought into engagement with the surface of a heated glass sheet. Thus, fiberglass is the material which is used to cover forming dies and this material, unfortunately, sticks to ceramic paint compositions then known to skilled artisans when those compositions are heated to an elevated temperature which is necessary to form glass by a forming die.
In my patent '590, I disclosed a method of forming glass sheets with a ceramic paint thereon which could withstand the high temperatures and repeated engagements of painted glass sheets by the same forming die without paint sticking to the forming die. I solved the problem by adding to the ceramic paint, as a new component thereof, a metal oxide having a low valence oxidation state and a high valence oxidation state. The metal oxide is in its low valence state when the ceramic paint is applied. My patent taught that I believed, when this metal oxide powder is subjected to a heating operation, that portion of the powder on the upper surface of the ceramic paint composition readily oxidizes to form an excellent, fully oxidized metal oxide barrier across the top surface of the paint which insulates the low melting point glass frit and isolates the same from the fiberglass cloth. In particular, I taught that stannous oxide was an excellent metallic oxide powder to use.
In carrying on additional work in this area, particularly in working with oil-base ceramic paints, I found that the utilization of stannous oxide did have some drawbacks. Stannous oxide has a tendency to oxidize rapidly when subjected to a heating operation. This rapid oxidation causes a fine, white dust to be generated which loosens from the surface of the paint and is moved about the glass sheet during the bending operation. The loose dust at the elevated temperature can cause marking and/or indentation of the glass sheet, which results in a scrappage thereof. This dusting problem was particularly severe when stannous oxide was used in conjunction with an oil-base ceramic paint.
No search was conducted on the subject matter of this specification in the U.S. Patent and Trademark Office or in any other search facility. The only prior art that I am aware of is my aforementioned patent.